In the field of document sorting, and in particular in relation to sorting mail pieces, there has been a need for an automatic sorter which will rapidly and efficiently sort mail pieces regardless of size and thickness whereby an individual mail carrier may sort his entire route by house number or doorway in a matter of minutes. This type of machine is capable of sorting, for example, three thousand pieces of mail in this fashion. Such a machine is described in copending U.S. patent application Ser. No. 07/961,980 filed Oct. 16, 1992, now U.S. Pat. No. 5,363,971 the disclosure of which is hereby incorporated by reference, and assigned to the U.S. Postal Service.
In that machine a plurality of vertical stackers are provided. The vertical stackers are fed by belts with individual mail pieces. The mail pieces are sequentially sorted so that in three passes through the device the random distribution of, for example, three thousand mail pieces will be sorted sequentially according to the carrier's route addresses. During the three passes each of the individual stackers is filled from the top and then emptied from the bottom. The roughly one thousand pieces of mail then are distributed preferably in 12 or 13 stackers which fill and empty three times in order to sequence the mail stream.
The stackers then are twenty-eight inches high and when fed from the top must permit the stacking of mail pieces on their flat surfaces and not on the edges. In order to achieve this, a paddle is disposed adjacent the top of the stacker, initially, and as mail pieces are fed into the stacker they rest on top of the paddle which lowers as the mail pieces accumulate. When the paddle reaches the bottom, it translates out from under the stack and returns to the top to receive mail sorted in the next pass. The stack is then emptied sequentially from the bottom for a subsequent round of sorting to fill the stacker again from the top. The paddle accordingly must translate vertically downwardly, horizontally to be withdrawn from the stack, vertically upwardly, and then horizontally into the stack. These motions must be smooth and achievable fairly rapidly as the entire machine will sort the three thousand pieces of mail in three passes within minutes.
Articulation of the paddle could be achieved with a linear actuator such as a ball screw device. In a ball screw device, a shaft which is threaded turns and an internally threaded ball nut or block mounted thereon supports the paddle. Therefore, movement of the ball nut or block vertically on one shaft and horizontally on a separate shaft would achieve the necessary articulation.
The ball screw type device in addition to being noisy, expensive and bulky, has a disadvantage in that it must be kept clean. If dirt accumulates in the threads, the device can jam which would require dismantling and cleaning. Accordingly, it was proposed to supply a separate bellows adjacent the threaded shaft to keep the dust from accumulating in the threads. This of course adds additional cost and an additional feature to the machine which itself could be subject to breakdown.